In modern business, economic, and scientific environments, color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of digital color document output devices are continuously searching for techniques, which can improve the accuracy and total image quality of their products.
Color images are commonly represented as one or more separations, each separation comprising a set of color density signals for a single primary color. Color density signals are commonly represented as digital pixels, which vary in magnitude from a minimum to a maximum, with a number of gradients corresponding to the bit density of the system. Thus, for example, a common 8-bit system can provide 256 shades of each primary color.
A color can therefore be considered as the combination of magnitudes of each pixel, which when viewed together present the combination color. Usually, printer signals include three subtractive primary color signals (i.e., Cyan (C), Magenta (M) and Yellow (Y)). Often a fourth Black (K) signal is also employed. Together, these primaries can be considered the printer colorant signals. Each color signal forms a separation, and when combined together with the other separations forms the color image.
It is desirable to specify document properties in a device-independent fashion in order to facilitate the exchange and reuse of documents where possible. Colors are therefore preferably specified in a device-independent color space based on the characteristics of human vision. In order to print or display a given color, it is often necessary to determine the device control values corresponding to specified device-independent color values, because the native control spaces of output devices (e.g., a printer's CMYK values) do not constitute device-independent color spaces. This normally can be accomplished utilizing a three-step procedure.
First, a set of color patches with pre-determined device control values is output on the device and the color of each patch is measured in device-independent color coordinates. Second, utilizing the device control values and the corresponding measured device-independent color values, a “forward device-response function” can be estimated. Third, the “forward device-response function” can be “inverted” to obtain a “device-correction-function”.
The “forward device-response function” of step two represents the mapping from device control values to the device independent color values produced by the device in response to the control values. The “device-correction-function” of step three maps each device-independent color to the device control values that produce the specified device-independent color value on the output device. The “device-correction-function” is typically pre-computed and stored in memory. In order to produce a given color on the output device, the corresponding device-independent color values are mapped through the “device correction-function” to obtain control values. When the device is driven with such control values, a desired color can be produced.
It is common practice to separate the “device correction-function” into two parts: a “calibration” function that immediately precedes the device and a “characterization” function, which addresses the device “through” the calibration function. This separation is illustrated in FIG. 1 for the case of a conventional CMYK printer. In FIG. 1, a conventional system 100 is depicted, which can be implemented as a CMYK printer. System 100 can be divided into a “device-correction function” 105 and a “calibrated device” portion 107. The “device correction function” 105 can be partitioned into characterization and calibration portions, respectively represented by a characterization routine 102 and a calibration unit 104.
A device independent color can be provided as input 110 to the characterization routine 102, whose output can be fed to a calibration unit 104. The output from calibration unit 104 can be provided in turn to an output device 106 as indicated by an output line 114. Additionally, line 112 indicates alternate CMYK (i.e., fast emulation). Data can be output from a reprint path unit 108 and fed to the calibration unit 104. In FIG. 1, the “calibration device” portion 107 of system 100 can be formed generally from calibration unit 104 and output device 106.
Another example of a device correction system includes U.S. Pat. No. 5,305,119 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Apr. 19, 1994 and is assigned to Xerox Corporation. U.S. Pat. No. 5,305,119 is generally directed toward a method of characterizing and calibrating a response of a printer to an image described in terms of colorimetric values. A further example of a device correction method and system is described in U.S. Pat. No. 5,528,386 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Jun. 18, 1996 and is also assigned to Xerox Corporation. U.S. Pat. No. 5,528,386 generally describes a conventional one-dimensional architecture for the calibration step. Both U.S. Pat. Nos. 5,305,119 and 5,528,386 are incorporated herein by reference.